Nodal is a transforming growth factor (TGF)-beta like protein that we discovered on the basis of a retroviral insertional mutation and showed to be essential for early development, playing a major role in gastrulation. Our recent work on nodal signaling in mouse embryonic development has focused on the analysis of a targeted mutant allele with reduced function (a hypomorph). Embryos that are compound heterozygotes for this hypomorphic allele and a completely null allele are capable of developing past the block at gastrulation seen in homozygotes for the null allele, but then show a variety of developmental abnormalities. This has allowed us to establish a critical role for nodal in developmental processes occurring after gastrulation begins, including patterning of the anterior/posterior and left/right body axes, and the development of the midline mesendoderm and gut endoderm. However, because there is a generalized reduction of activity in each of the many domains of nodal expression our analysis did not provide insight into where nodal expression is required for these developmental events. To dissect these various temporal and spatial domains of nodal function further, we generated a conditional mutant (floxed) allele. Mice carrying this allele are being used in conjunction with transgenic strains expressing Cre recombinase in various regions of the developing embryo, which we have developed or obtained from other investigators. Preliminary analysis of embryos obtained from breeding floxed mice with a strain expressing Cre in the pregastrulation embryonic epiblast, the cells that will give rise to all the germ layers, has revealed all of the same phenotypes observed earlier for the hypomorphic allele. The possibility that expression in extraembryonic lineages may not be required for early nodal function is now being addressed by the use of a strain expressing Cre specifically in the extraembryonic visceral endoderm. Additional ongoing work is being performed with Cre strains with other tissue specificities.